Weathering trends in terrace deposits up to 350,000 years old in northeast Southland, New Zealand

Gravelly floodplain deposits and six terrace treads in the Waimea Plains, northeast Southland, New Zealand, range in age from ca. 1000 years old to 350,000 yr old. Deposits are progressively more weathered with increasing parent material age. Soils developed in them are classified as Recent Soils, Brown Soils and Ultic Soils in the New Zealand Soil Classification and Udifluvents, Dystrochrepts, Haplohumults, Hapludults and Paleudults in the USDA Soil Classification (Soil Taxonomy). The youngest soils are slightly acid. Those developed in deposits 35,000-70,000 yr old are strongly acid. In soils in deposits 160,000 yr old and older, that have weathered in two or three interglacial periods, including the present one, pH and total exchangeable bases values are higher than in soils in younger deposits. The higher pH of the soils in older deposits is attributed to release of cations by stone weathering. Soils developed in deposits up to 70,000 yr old have clays dominated by mica and chlorite. Stones in these soils are at most weakly weathered, and the < 2 mm fraction contains 2-13% day and 63-95% sand. Weathering of older deposits has destroyed the fabric of most stones. The < 2 mm fraction of soils developed in deposits 250,000-350,000 yr old contains 34-38% clay and 20-24% sand; clays are dominated by kaolin-group and mica-smectite minerals and 60% of the total Fe in the < 2 mm Fraction is in secondary forms. Two phases of weathering are distinguished: a phase of approximate duration 100,000 yr in which most weathering and leaching occurs in the matrix, which becomes increasingly acidified; and a phase from 100,000 to 350,000 yr which is dominated by cation release from stones, reversal of the acidification trend, and breakdown of stone fabric.